Solid waste is divided up from various categories, for instance, based on its origin into so-called city waste, industrial waste and agricultural waste, or, based on its combustibility into flammable and non-flammable waste, or, based on its composting capacity, into waste usable or unusable for compost.
Waste materials contain important natural materials which, and this is an economic necessity, should be recycled into the economic cycle, and that either directly, by which the organic materials and minerals are fed into the soil, or indirectly, by using waste materials as replacement for organic materials and minerals which, otherwise, would have to be taken from the soil.
An essential element of the presented invention employs a generally known composting method, namely, biological oxydation of waste material under aerobic conditions.
Aerobic biological oxydation is a process which is dependent on the available quantity of carbon, i.e., of the carbon in the compost mass that is available in a micro-organisms as food and energy supplier. In this composting process the most readily available carbon portions are those from sugars, starch portions, proteins and hemicelluloses, and in this sequence, are, the first to be degraded or broken up. Subsequently, to a certain extent the cellulose is degraded. The Lignines which, in the sense of this invention, offer almost no available carbon to be degraded are of no importance.
We then distinguish, in principle, in the sense of the invention, the so-called available carbon, i.e., carbon that can be used by micro-organisms in the composting process, and such carbon that cannot be used in the biological process.
Several parameters exist that influence and distinguish the composting processes, especially (a) the extent and duration of aeration, (b) the composition of the surface and the particle size of the raw material to be turned into compost, (c) the moisture content of the mass and, finally, (d) the pH-value of the mass to be turned into compost.
The exothermally developing heat of the compost mass and the carbon dioxide production are not established parameters of the oxydation, but they are indicator values of the development of the composting process.
The present invention rests on the recognition that the speed of the composting process, that is the speed of the waste conversion, in other words, the progress from the mesophilic phase to the thermophilic phase and the extent of the composting, namely, the amount of available carbon that is used by micro-organisms, can be controlled only, if each of the above mentioned factors (a), (b), (c), (d) can be influenced individually and independently from each other, throughout the duration of the process.
It is, therefore, the task of the present invention to obtain, by continuous individual control and influence of all factors important for the composting process, a stable, uniform material with predetermined biological, chemical, physical and mechanical characteristics that can be processed further.
This task cannot be solved by the composting processes known up to now.
The so-called Windrow-process, as well as the so-called Batch-process, are already lacking this control possibility because of their uniqueness.
The continuous processes, for instance, those using the slightly slanted horizontal rotating Dano-drums, and those, using the vertically arranged so-called Multibacto-cylinders, permit only the control of the input and output quantities.
The stability of the material, in the Windrow as well as in the Batch processes (the latter are discontinuous processes) cannot be controlled at all; in the continuous flow processes mentioned, the outcoming material must be cured for a longer period of time, before it reaches a somewhat satisfactory stability can be reached.
In creating a stable and uniformly workable material, as set forth in the present invention, three aspects are of great importance. Based on the invention as presented, the uniform quality of the output material is independent from the composition of the incoming waste materials. This is especially important in view of the constantly rising percentage of paper content in city wastes. 15 - 20 years ago, the paper portion in city waste was, in industrialized countries, approximately 25-30%. However, presently that portion is approximately 50%, and in some metropolitan areas is approximately 60% and more. The composting processes mentioned, based on the present state of technology (e.g., Windrow process, Batch process) are only capable of composting waste with a portion of approximately 30% paper. Any amount above this figure must be removed before being put into the system.
In contrast thereto, in the composting of city waste it is, nowadays, a foremost requirement that the compost plant be capable of accepting and transforming the waste in the manner in which it is delivered without regard to composition. The necessity of removing an essential amount of paper from the delivered waste would present an unreasonable burden on the economics of the process.
From public health and economic viewpoints, the major presence of putrescible materials, in the composting mass is a major problem. A radical elimination of all putrescible materials contained therein would only be possible by burning, which not only destroys the putrescible materials, but also destroys the cellulose which is one of the most important natural contents of the waste masses. In contrast, biological oxydation, namely composting, make it possible to clean the cellulose, which can be stabilized, from the putrescible materials.
Such a cleaning can be achieved by adhering to the aforementioned control factors (a), (b), (c), and (d) of the process set forth in the present invention.
Depending on the desired characteristics of the output, optimal combinations of the factors mentioned before are possible and necessary.
If, for instance, the workable compost product is to be used as a carrier in enriched granulated organic fertilizers, the biological characteristics and, especially, the stability, are of the greatest importance, whereas the mechanical characteristics, as, for instance, the size of the particles, are of lesser importance. If, however, the workable composted material is to be used as basis for the manufacture of fiber products (fiber board, etc.), the stability is less important while, in this case, the size of the particle, for instance, the length of the fiber, is the most important factor.